Mercury is a pervasive global pollutant which bioaccumulates in the food web and is highly toxic to humans and other organisms. Anaerobic microorganisms, such as sulfate-reducing bacteria (SRB), iron-reducing bacteria and methanogens have been implicated as producers of methylmercury (MeHg). However, phylogenetic analyses based on 16S sequences cannot distinguish between methylating and non-methylating microorganisms (Ranchou-Peyruse 2009; Gilmour 2011; Yu et al. 2012).
Biological mercury methylation was shown to be an enzyme-catalyzed process and proposed to be associated with the reductive acetyl-CoA (Wood-Ljungdahl) pathway (Wood 1968; Choi 1994b). A 40 kDa corrinoid-binding protein capable of methylating mercury was identified in cell extracts of the sulfate-reducing bacterium Desulfovibrio desulfuricans LS—a methylating SRB strain. Unfortunately, the strain was lost and further characterization of that 40 kD protein is not possible (Gilmour 2011). Although mercury methylation activity was proposed to be associated with the reductive acetyl-CoA (Wood-Ljungdahl) biochemical pathway (Choi et al. 1994a), no consistent relationship was ever established between that pathway and the ability to methylate Hg(II), suggesting the existence of an alternative, but as yet unidentified, pathway or pathways to form MeHg in microorganisms (Ekstrom 2003).
Accordingly, the need remained to understand the genetic and biochemical basis for microbial mercury methylation and to identify specific microorganisms with the potential to methylate mercury. The availability of the complete genome sequences for known mercury methylators and non-methylators aided the discovery, identification and characterization of two genes and their gene products, associated with a mercury methylation pathway common to all known methylating bacteria and archaea sequenced to date. Characterization of these gene products explains mercury methylation in microorganisms whether or not the complete acetyl-CoA pathway is present in those microorganisms. Moreover, the availability of the genes provides a biomarker for microbial mercury methylation that can be used to identify methylators as well as to assess the mercury methylation potential.